1. Field of the Invention
The present invention generally relates to switching regulators, and particularly relates to a switching regulator which supplies a plurality of DC power supply voltages.
2. Description of the Related Art
In recent years, switching regulators have been used in a variety of electrical equipment, resulting in a demand for a low-cost switching regulator occupying a small space.
FIG. 1 is a circuit diagram showing a related-art switching regulator of a synchronous rectification type that can produce two output voltages higher than a input power potential.
The switching regulator of FIG. 1 is coupled to a DC power supply 1, and includes an inductor L1 for OUT1, a semiconductor switch SW1 for providing a current to the inductor L1 for OUT1, a rectifying diode 4 for OUT1, a semiconductor switch SW2 for OUT1, a rectifying smoothing condenser 6 for OUT1, an inductor L2 for OUT2, a semiconductor switch SW3 for providing a current to the inductor L2 for OUT2, a rectifying diode 10 for OUT2, a semiconductor switch SW4 for OUT2, a rectifying smoothing condenser 12 for OUT2, and timing control circuits 14 and 15. An output OUT1 appearing between the opposite ends of the rectifying something condenser 6 is supplied to a load 7. An output OUT2 appearing between the opposite ends of the rectifying smoothing condenser 12 is supplied to a load 13.
FIG. 2 is a timing chart for explaining the operation of the switching regulator shown in FIG. 1.
In FIG. 2, the switches SW1, SW2, SW3, and SW4 are closed (i.e., in a conductive state) during the HIGH period of respective timing control signals, and are open (i.e., in a nonconductive state) during the LOW period of the respective timing control signals. In the following, the operation of the circuit shown in FIG. 1 will be described with respect to the output OUT1.
When the switch SW1 is closed for a time t1 while the switch SW2 is open, an electric current is supplied from the DC power supply 1 to the inductor L1, resulting in the inductor L1 accumulating energy that is proportional to the square of the time t1. The accumulation of energy can be observed as the flow of an electric current running through the inductor as shown in FIG. 2(c).
The switch SW1 is then opened, immediately followed by closing the switch SW2 for a time t2. The energy accumulated in the inductor L1 is discharged through the switch SW2 (and the diode 4), moving to the condenser 6. As a result, the condenser 6 stores energy therein as electric charge, resulting in an increase in the terminal voltage OUT1.
After the end of the time t2, both the switches SW1 and SW2 are kept open, so that an electric current runs from the condenser 6 to the load 7. Until the switches SW1 and SW2 operate again (corresponding to a time t3), the energy of the condenser 6 continues to discharge, so that the terminal voltage OUT1 (FIG. 2(d)) decreases with time. Here, the voltage waveform shown in FIG. 2(d) illustrates an enlarged view of minute voltage changes.
The operations described above are repeated. When a certain operation state is achieved in which the energy stored in the condenser 6 matches the energy discharged, electric charge discharged from the condenser 6 is constantly replenished by the subsequent building up of charge. As a result, a direct current potential is obtained as the output OUT1.
The timing control circuit 14 compares the direct current potential of the output OUT1 with a predetermined potential. The timing control circuit 14 controls the switching timing of the switch SW1 to shorten the time t1 if the DC potential of the output OUT1 is higher, and to elongate the time t1 if the DC potential of the output OUT1 is lower. In the case of a PWM (pulse width modulation) method having a variable t1, a total of the time t1, the time t2, and the time t3 is constant, as determined by the clock frequency selected by the timing control circuit 14.
In the construction of FIG. 1, the switch SW2 may be removed, with only the rectifying diode 4 being in its place. In a silicon diode, however, a potential drop of approximately 0.6 V is generally generated when an electric current more than a few mA runs in the forward direction. Such a potential drop creates energy loss. When energy efficiency is of primary concern, therefore, the semiconductor switch SW2 is used that has a small ON resistance creating a lower potential drop than the diode. If only the semiconductor switch SW2 is used, however, it is possible that the switch SW2 is opened while some energy remains in the inductor L1. When this happens, the inductor L1 generates a high potential, which may destroy the circuit. Because of this, it is preferable to provide the diode 4 in parallel to the semiconductor switch SW2 as shown in FIG. 1 for the purpose of preventing the generation of such high potential.
The operations as described above are carried out with respect to the output OUT2 in the same manner.
In the construction of FIG. 1, the circuitry for the output OUT1 and the circuitry for the output OUT2 are separately provided, so that circuit components are provided in duplicate for both outputs. In a related-art switching regulator having a plurality of DC outputs, generally, circuit components such as an inductor, a diode, a semiconductor switch, and a condenser need to be provided as many as there are outputs. This results in a cost increase and also an increase in circuit size. An inductor is a circuit component that cannot easily be reduced in size, which hampers an effort to reduce costs and size.
Accordingly, there is a need for a switching regulator circuit which is reduced in costs and size.